Geophysical inversion attempts to find a model of subsurface properties that optimally explains observed data and satisfies geological and geophysical constraints. There are a large number of well known methods of geophysical inversion. These well known methods fall into one of two categories, iterative inversion and non-iterative inversion. The following are definitions of what is commonly meant by each of the two categories:                a. Non-iterative inversion—inversion that is accomplished by assuming some simple background model and updating the model based on the input data. This method does not use the updated model as input to another step of inversion. For the case of seismic data these methods are commonly referred to as imaging, migration, diffraction tomography or Born inversion.        b. Iterative inversion—inversion involving repetitious improvement of the subsurface properties model such that a model is found that satisfactorily explains the observed data. If the inversion converges, then the final model will better explain the observed data and will more closely approximate the actual subsurface properties. Iterative inversion usually produces a more accurate model than non-iterative inversion, but is much more expensive to compute.        
In general, inversion is beneficial in correcting observed seismic data so that reflections are plotted at a true representation of their subsurface locations (Stolt, 1978; Claerbout, 1985). The need to correct (i.e., invert) observed seismic data arises, for example, because reflections from dipping interfaces are observed, and therefore recorded, at surface positions that are not directly above the subsurface locations where the reflections actually occur. Also, isolated point-like discontinuities in the subsurface (i.e., point scatterers) generate seismic events (e.g., diffractions) recorded over a large range of receivers. Such diffractions can make the proper interpretation of seismic data confusing. Furthermore, seismic velocity variations can also cause a distorted view of the subsurface geology (Yilmaz, 1987). It is only after inversion that the structures and geometric configurations observed in seismic recordings can be thought of as an accurate depiction of the geologic layers that gave rise to the seismic reflections.
Because of the increased complexity of iterative inversion, it is generally desirable to use a non-iterative form of inversion (i.e., imaging, migration, etc.) when possible. However, as industry explores more complex geographic areas, traditional imaging and interpretation methods fail to provide subsurface images having the quality (e.g., accuracy) desired in making decisions on exploration and production. For example, wave-equation migration algorithms are based upon the one-way wave-equation approach. The one-way wave equation assumes that waves propagate in only one primary direction, either down into the subsurface or up from the subsurface. Because of the one-directional nature of propagation, imaging steeply dipping reflectors is difficult.
Advanced imaging tools that use full physics of wave-propagation, such as reverse time migration (i.e., RTM), generally provide better images of the subsurface. Such approaches use solutions of the two-way wave equation. Migration techniques that use the two-way wave equation generally provide a more accurate result because waves propagating in all directions are handled equally well, and wave amplitudes are properly modeled since no approximations are used in the algorithm. However, there is a cost associated with conventional two-way wave equation techniques. Specifically, the full physics of propagating waves in a complex geological setting where the medium velocity is complicated is very computationally intensive. The computational demand is further accentuated when there are many (e.g., thousands) shot records to be migrated in a three-dimensional (3D) setting and/or when high frequency data is obtained in an effort to increase the resolution of the subsurface images. It is desirable, therefore, to have a system and/or method for increasing the computational efficiency (i.e., reducing the computational cost) of two-way wave equations based techniques, such as RTM.